1. Field of the Invention
The present invention relates to a process for preparing alcohols by reacting carboxylic acids and/or carboxylic esters with hydrogen in the presence of a special catalyst, and to such catalysts.
2. Brief Description of the Prior Art
The hydrogenation of carboxylic acids and carboxylic esters in the presence of catalysts based on copper chromite (Adkins catalyst) has been known for some time. However, the use of chromium-containing catalysts is undesirable for reasons of the associated risks to the environment. Efforts have therefore been made to replace these chromium-containing catalysts by more environmentally friendly chromium-free catalysts.
For instance, WO 82/03854 Al discloses a process for hydrogenating carboxylic esters in the presence of a catalyst which comprises a reduced mixture of copper oxide and zinc oxide.
EP-A 0 721 928 describes a process for preparing aliphatic α,ω-diols by hydrogenating carboxylic esters using a catalyst which comprises a reduced mixture of pressed powders of copper oxides, zinc oxides and aluminium oxides, to which iron oxide, nickel oxide or manganese oxide may be added.
U.S. Pat. No. 5,155,086 describes pulverulent hydrogenation catalysts based on copper/zinc/aluminium which contain a majority of oxides of copper and zinc, and relatively small amounts of aluminium oxide, and the pore volume of the pores which have a diameter of 120 to 1000 Å is at least 40% of the total pore volume. In particular the catalysts are suitable for hydrogenating aldehydes, ketones, carboxylic acids and carboxylic esters.
Catalysts based on copper/zinc/aluminium are known from methanol synthesis (U.S. Pat. No. 4,305,842, EP 125 689 A2). In these catalysts, aluminium oxide and zinc oxide have the function of a support material for copper. Such catalysts are prepared, for example, by coprecipitating the components and converting to active catalysts by calcining and reducing (Knözinger, Ertl, Weitkamp, Handbook of Catalysis, VCH Wiley, Weinheim 1997, 1836).
Hydrogenation catalysts based on copper/zinc/aluminium are also known in which colloidal TiO2 or Al(OH)3 have been added to the product of coprecipitating copper and zinc (EP 125 689 A2, Petrini et al., Preparation of catalysts III, Studies in surface science and catalysis, 16, Elsevier, Amsterdam, 1983, 735–755.)
JP J09-173845 describes the preparation of Cu/Zn catalysts which are prepared by saturating γ-aluminium oxides and have an aluminium content of approx. 53%, and the use of such catalysts in the synthesis of dimethyl ether.
WO 99/4974 describes catalysts which are prepared by precipitating Cu and Zn onto TiO2. To prepare the tablets from the pulverulent catalyst, metallic copper powder is added as a tableting assistant in order to achieve a sufficient hardness. DE 19942895 likewise describes the effectiveness of metallic copper or cement as tableting assistants for preparing Cu/Zn/Al catalysts for hydrogenating carboxylic esters. This addition also leads to an increase in the lateral fracture hardness.
WO 97/34694 describes Cu/Al/Zn catalysts having an Al content of greater than 20% which, in the form of extrudates, have a bimodal pore size distribution. These catalysts are very suitable for the hydrogenation of fatty esters.
When used in fixed bed reactors, the existing catalysts are used as shaped bodies which only have limited mechanical stability under the mechanical stresses occurring there. In addition, the hydrogenation activity of these catalysts, particularly in the hydrogenation of esters of polybasic acids with polyhydric alcohols, for example mixtures of oligomeric esters made from adipic acid and hexanediol, is insufficient for the achievement of high space-time yields.
It is therefore an object of the invention to provide a process for preparing alcohols by reacting carboxylic acids and/or carboxylic esters with hydrogen in the presence of a catalyst which is notable for its high mechanical stability under the reaction conditions and a high activity so that relatively high space-time yields are achieved.